Chapter 62: General Principles of GI
Function—Motility, Nervous Control,
and Blood Circulation
Guyton and Hall, Textbook of Medical Physiology, 12th edition
Alimentary Tract
• Provides the Body with Water, Nutrients, Electrolytes,
and Vitamins By:
a. Movement of food through the alimentary tract
b. Secretion of digestive juices and digestion of the food
c. Absorption of water, various electrolytes, vitamins,
and digestive products
d. Circulation of blood through the GI organs to carry
away the absorbed substances
e. Control of all these functions by local, nervous, and
hormonal systems
Alimentary Tract
Fig. 62.1
General Principles of GI Motility
• Physiologic Anatomy of the GI Wall- Layers from
the outer to inner
a.
b.
c.
d.
e.
Serosa
Longitudinal smooth muscle layer
Circular smooth muscle layer
Submucosa
Mucosa
General Principles of GI Motility
• Physiologic Anatomy of the GI Wall
Fig. 62.2 Typical cross section of the gut
General Principles of GI Motility
• GI Smooth Muscle Functions As a Syncytium
a. Individual smooth muscle fibers are 200-500 um
in length, 2-10 um in diameter, and arranged in
bundles containing as many as 1000 fibers
b. Fibers are electrically connected through large
numbers of gap junctions allowing rapid movement
of electrical signals for contraction
c. Muscle bundles fuse with each other at many points
so in reality each layer is a branching latticework
of smooth muscle bundles
General Principles of GI Motility
• GI Smooth Muscle Functions As a Syncytium
d. When an AP is elicited anywhere within the
muscle mass, it generally travels in all directions
•
Electrical Activity of GI Smooth Muscle
a. Slow waves-most GI contractions occur rhythmically,
and this is determined mainly by the frequency of
slow-waves of smooth muscle
General Principles of GI Motility
Fig. 62.3 Membrane potentials in intestinal smooth muscle.
General Principles of GI Motility
• Slow Waves
a. Not APs, but slow undulating changes in the
resting membrane potential
b. Appear to be caused by interactions between
smooth muscle cells and the interstitial cells of
Cajal (act as electrical pacemakers for smooth
muscle cells
c. Do not cause muscle contraction by themselves
but excite the appearance of intermittent spike
potentials, which then excite the muscle
General Principles of GI Motility
• Spike Potentials
a. True action potentials
b. Occur automatically when the resting
membrane potential of the GI smooth muscle
becomes more positive than -40 mV.
c. Last 10-40X as long in GI smooth muscle as in
large nerve fibers
General Principles of GI Motility
• Spike Potentials
d. Channels responsible are calcium-sodium channels
e. Channels are much slower to open and close than
those of nerves
General Principles of GI Motility
• Changes in Voltage of the Resting Membrane
Potential
a. Under normal conditions the resting potential is
-56 mV
b. Factors that depolarize
1. Stretching of the muscle
2. Stimulation by AcH (parasympathetic)
3. Stimulation by specific GI hormones
General Principles of GI Motility
• Changes in Voltage of the Resting Membrane
Potential
c. Factors that hyperpolarize
1. Effect of epinephrine or norepinephrine
2. Stimulation of sympathetic nerves that
secrete mainly norepinephrine
General Principles of GI Motility
• Calcium Ions and Muscle Contraction
a. Calcium ion, acting through a calmodulin mechanism
activate the myosin fibers, causing interaction with
the actin fibers to initiate contraction
b. Slow waves do not cause calcium ions to enter the
smooth muscle fiber (only sodium)-so no contraction
c. Spike potentials allow significant calcium to enter
and cause most of the contraction
General Principles of GI Motility
• Tonic Contraction of Some GI Smooth Muscle
a. Tonic contraction is continuous and not
associated with the basic electrical rhythm of
the slow waves
b. Sometimes caused by continuous repetitive
spike potentials
c. Can be caused by hormones
d. Continuous entry of calcium in ways not associated
with changes in membrane potential
Neural Control of GI Function-Enteric Nervous System
• Enteric Nervous System
a. Lies entirely within the wall of the gut
b. Composed of 100 million neurons
c. Composed of mainly two plexuses
1. Myenteric plexus-outer plexus between the
longitudinal and circular muscle layers
2. Submucosal pleuus-lies in the submucosa
Neural Control of GI Function-Enteric Nervous System
• Enteric Nervous System
Fig. 62.4
Neural Control of GI Function-Enteric Nervous System
d. Sensory nerve endings that originate in the GI wall or
epithelium send afferent fibers to both plexuses as well
as
1. Prevertebral ganglia of the sympathetic system
2. To the spinal cord
3. In the vagus nerves all the way to the brain stem
Neural Control of GI Function-Enteric Nervous System
•
Differences Between the Myenteric and
Submucosal Plexuses
a. Stimulation of myenteric plexus causes
1. Increased tonic contraction of the gut wall
2. Increased intensity of rhythmical contractions
3. Slightly increased rate of the rhythm of
contraction
4. Increased velocity of conduction of excitatory
waves along the gut wall
Neural Control of GI Function-Enteric Nervous System
•
Differences Between the Myenteric and
Submucosal Plexuses
b. Some neurons of the myenteric are inhibitory
c. Submucosal plexus
1. Mainly concerned with controlling function within
the inner wall of each minute segment of the
intestine
Neural Control of GI Function-Enteric Nervous System
•
Types of Neurotransmitters Secreted by Enteric
Neurons
1. Acetylcholine-most often excitatory
2. Norepinephrine and epinephrine-most often
inhibitory
3. ATP
4. Serotonin
5. Dopamine
6. CCK
7. Substance P
8. Somatostatin
9. Enkephalins
Neural Control of GI Function-Enteric Nervous System
•
Autonomic Control of the GI Tract
a. Parasympathetic stimulation increases activity of
the Enteric Nervous System
b. Sympathetic stimulation usually inhibits GI tract
activity
1. By the direct effect of norepinephrine of smooth
muscle
2. By the inhibitory effects of norepinephrine on
the neurons of the Enteric Nervous System
Neural Control of GI Function-Enteric Nervous System
•
Afferent Sensory Nerve Fibers From the Gutcell bodies may be in the Enteric Nervous System or
in the dorsal root ganglia of the spinal cord;
stimulated by
1. Irritation of the gut mucosa
2. Excessive distension of the gut
3. Presence of specific chemicals in the gut
Neural Control of GI Function-Enteric Nervous System
•
Gastrointestinal Reflexes
a. Reflexes that are integrated entirely within the gut
wall enteric nervous system
b. Reflexes from the gut to the prevertebral sympathetic
ganglia and then back to the GI tract
c. Reflexes from the gut to the spinal cord or brain stem
and back to the GI tract
Neural Control of GI Function-Enteric Nervous System
•
Hormonal Control of GI Motility (Table 62.1)
Hormone
Stimulus for
Secretion
Site of Secretion
Actions
Gastrin
Protein, Distension,
Nerve (acid inhibits
release)
G cells of the antrum,
duodenum, and
jejunum
Stimulates gastric acid
secretion and mucosal
growth
CCK
Protein, Fat, Acid
I cells of the small
intestine
Stimulates pancreatic
secretions, gallbladder
contraction and growth of
exocrine pancreas. Inhibits
gastric emptying
Secretin
Acid, Fat
S cells of the small
intestine
Stimulates pepsin secretion
and bicarbonate secretion,
growth of exocrine
pancreas. Inhibits gastric
acid secretion
Gastric Inhibitory
Peptide (GIP)
Protein, Fat,
Carbohydrate
K cells of the
duodenum and
jejunum
Stimulates insulin release
and inhibits gastric acid
secretion
Motilin
Fat, Acid, Nerve
M cells of the
duodenum and
jejunum
Stimulates gastric motility
and intestinal motility
Functional Types of Movements in the GI Tract
•
Propulsive Movements-Peristalsis
Fig. 62.5 Peristalsis
Functional Types of Movements in the GI Tract
•
Propulsive Movements-Peristalsis
a. Usual stimulus is distension of the gut
b. Other stimuli can include chemical or physical
irritation of the gut or strong parasympathetic
stimulation
c. Function of the myenteric plexus-effectual
peristalsis requires a functional myenteric plexus
Functional Types of Movements in the GI Tract
•
Propulsive Movements-Peristalsis
d. Directional movement of peristaltic waves is
toward the anus
e. Peristaltic Reflex and the “Law of the Gut”alternating contraction and relaxation as peristalsis
occurs; the peristaltic reflex plus the direction of
movement is called the law of the gut
Functional Types of Movements in the GI Tract
•
Mixing Movements
a. Differ in different parts of the alimentary tract
b. Other than typical peristalsis, there is local
intermittent constrictive contractions
c. also, if peristalsis is blocked by a sphincter then
only churning occurs
Gastrointestinal Blood Flow- Splanchnic Circulation
•
Mixing Movements
a. Differ in different parts of the alimentary tract
b. Other than typical peristalsis, there is local
intermittent constrictive contractions
c. Also, if peristalsis is blocked by a sphincter then
only churning occurs
Gastrointestinal Blood Flow- Splanchnic Circulation
Fig. 62.6 Splanchnic circulation
Gastrointestinal Blood Flow- Splanchnic Circulation
• Anatomy of the GI Blood Supply
Fig. 62.7 Arterial blood supply to the intestines through the mesenteric web
Gastrointestinal Blood Flow- Splanchnic Circulation
• Effect of Gut Activity and Metabolic Factors on GI
Blood Flow
a. Blood flow in each area of the GI tract and layers of
the gut wall is directly related to the level of local
activity
b. Causes of increased blood flow during GI activity
1. Vasodilators released from the mucosa of the
intestinal tract during digestion (CCK, gastrin,
secretin, vasoactive intestinal peptide)
Gastrointestinal Blood Flow- Splanchnic Circulation
2. Release of kallidin and bradykinin
3. Decreased oxygen cocentration in the gut wall;
decrease in oxygen can lead to a fourfold increase
in adenosne (vasodilator)
Gastrointestinal Blood Flow- Splanchnic Circulation
•
Countercurrent Blood Flow in the Villi
Fig. 62.8
Gastrointestinal Blood Flow- Splanchnic Circulation
•
Nervous Control of GI Blood Flow
a. Parasympathetic nerves increase local blood flow
and increases glandular secretion
b. Sympathetic causes intense vasoconstriction of
the arterioles and decreases blood flow
c. Sympathetic vasoconstriction allows skeletal
muscle and the heart to get extra flow when
needed